The present invention relates generally to obtaining accurate optical emission spectroscopy measurements. More particularly, the present invention relates to a system and method for radiometric calibration of spectroscopy equipment utilized in fault detection and process monitoring.
In the art of semiconductor processing, in order to form integrated circuit structures from wafers, selectively removing or depositing materials on a semiconductor wafer is well known. Removal of material from a semiconductor wafer is accomplished by employing some type of etching process, for instance and including, reactive ion etching, deep-ion etching, sputtering etching, and plasma etching. Depositing material on a wafer may involve chemical and physical vapor depositions, evaporative deposition, electron beam physical vapor deposition, sputtering deposition, pulsed laser deposition, molecular beam epitaxy and high velocity oxygen deposition. Other removal and deposition processes are known. Such processes are tightly controlled and are performed in a sealed process chamber. Because exact amounts of material are deposited onto or removed from the substrate wafer, its progress must be continually and accurately monitored to precisely determine the stopping time or endpoint of a particular process. Optically monitoring the chamber process is one very useful tool for determining the stage or endpoint for an ongoing process. For instance, the interior of the chamber may be optically monitored for certain known emission lines by spectrally analyzing predetermined wavelengths of light emitted or reflected from the target in the chamber. Conventional methods include optical emission spectroscopy (OES), absorption spectroscopy, reflectometry, etc. Typically, an optical sensor or source is positioned on the exterior of the chamber and adjacent to a viewport or window, with a vantage point to the target area in the chamber to be observed.
One problem with optical monitoring chamber processes is that it is difficult or impossible to accurately measure absolute values during many of these processes. This is primarily due to the accumulation of contaminants in the optical path, e.g., the clouding of the viewport windows from which optical measurements are made. Therefore, calibration processes known in the prior art have, to a large extent, evolved primarily in view of these unresolved problems. While it is possible to calibrate a spectrograph and its associated spectrographic detector across its entire spectral range using a broad band calibration standard, that level of accuracy is sometimes considered excessive since the viewport window will begin to cloud almost immediately, thereby reducing the accuracy of subsequent optical measurements. As the optical viewport window becomes clouded, it is sometimes presumed that its transmission is affected approximately uniformly across the entire spectral range of the spectrograph. Thus, many of the window clouding shortcomings can be compensated for somewhat by not relying on absolute values in the process and diagnostic algorithms. Thus, many measurement processes utilize comparisons of relative values rather than comparisons of absolute values. The prior art emphasizes the accuracy of the measurement of the particular spectra that are associated with a process gas and the accuracy of the measurement of the effect of contaminants on the viewport window.
U.S. Pat. No. 5,835,230 to McAndrew, et al. entitled “Method for Calibration of a Spectroscopic Sensor” discloses a system that utilizes a measurement cell with at least one light port (or a light entry port and a light exit port) with a light transmissive window through which a light beam passes along an internal light path inside the measurement cell. The calibration system also has an optical chamber which contains a light source for generating the light beam which passes through the light entry port into the cell as well as a detector for measuring the light beam exiting the cell through the light exit port. A gas inlet is connected to the optical chamber in which a calibration gas stream, that contains gas species and a carrier gas in known concentration, is introduced into the optical chamber. A spectroscopy measurement of the calibration gas stream is then performed. Using the calibration system, spectral calibrations can be realized for the spectrograph relative to a specific gas species and carrier gases in various concentrations.
U.S. Pat. No. 6,246,473 to Smith, et al. entitled “Method and Apparatus for Monitoring Plasma Processing Operations,” discloses an apparatus and calibration scheme for in situ measurements of the inner and outer surfaces of the viewport window in a plasma chamber. The apparatus includes a window monitoring or calibration module to determine the effect, if any, that the inner surface of the window is having on the light being emitted from the processing chamber during plasma processes. The calibration is intended to address wavelength shifts, intensity shifts, or both, that are associated with optical emissions data obtained on a plasma process. Essentially, the calibration device has dual optical paths, one path for optically monitoring light emitted from inside the process chamber, through the window, and another path for obtaining light reflected from calibration light sources for evaluating the state of the inner surface of the window. The calibration light source (or light sources) is located externally and projects a calibrated light that is reflected off of the surfaces of the window for making transmission comparisons for the window. Both U.S. Pat. Nos. 5,835,230 and 6,246,473 are incorporated by reference herein in their entireties.
Among other deficiencies, neither of these references addresses problems associated with calibrating the system along the entire optical path, from inside the chamber to the spectrographic sensor. Furthermore, the prior art calibration techniques rely heavily on the use of a local primary standard calibration light source.